Really great question! But I think it's all relative, depending on:
* hair length
* presence or absence of chemical treatments to hair (coloring, highlighting, relaxing, etc)
* use of heat styling
* the performance of regular shampoo and/or conditioner
* the performance of other styling products that may precede or follow the application of the leave-in
* how "heavy" the leave-in itself is on your hair
* any other "damage" to hair that we might be trying to help hide or "repair" with the leave-in.
As someone with color-processed hair (for lotsa years), my hair tends to be drier, so I use what I think is a "ton" of leave-in conditioner (about 3-4 quarter-sized globs of it). Even though I'm just around shoulder-length, I use more than a lot of people with much longer hair than mine. I know we all just do what works best for our hair, but that is always funny to me -- how much leave-in I use compared to really long-haired curlies (like Rain, for example).
Type: 3b/3c (i), below shoulder length. Modified CG since 5/2004; growing gray since 5/2007.
CO wash: A-O GPB LYY, Salon Care Honey Almond, Devacurl OC
Leave-ins: BWC, KCKT, jojoba oil overnight
Styling: BRHG, MGA Sculpting Gel, Aubrey Organics Mandarin Magic Gel, LOTD AVG, LALSG
DT: Terax Crema, Moroccan Oil Restorative Mask, Je Veux Mud Mask